Flat roof systems are a misnomer because they are not exactly flat. These roofing systems are intended for the roofs of larger buildings that do not have an obvious slope to drain water away and thus reduce potential leakage. A second disadvantage to these single-ply systems, especially in high wind areas, is their vulnerability to winds that can rip off roof covers that are not properly protected. Some flat roof systems are designed with special funnels designed to prevent this problem. The most common of the flat roof systems are the five described below.
1- Roll Asphalt
Material from which this system is comprised gets its name from some of the material used and the way it is applied to the roof it covers. The material, usually organic felt, or fiberglass felt, is saturated in asphalt and rolled onto the roof's surface. Although it lies flat against the roof in normal conditions, it is vulnerable to even light winds and could be easily blown off if it were not attached solidly to the roof or weighted down. Typically, it is attached by nailing it to the roof's substrate (usually a wood surface) or glued with cold asphalt cement, and it is weighted down by covering the asphalt layer with heavy material, such as gravel.
Like rolled asphalt roof systems, this system's name comes from the material from which the system is made, usually plastics such as thermoplastic polyolifin (TPO), or polyvinyl chloride (PVC). Both of these systems are usually good at reflecting heat and for this reason are often the preferred system in hotter climates where they can reduce cooling or air conditioning costs. Another system that reflects heat and reduces cooling costs is one that features a white ethylene propylene dience monomer (EPDM) material. Rubberized asphalt is a third type of single-ply system. Most of these single-ply roofs are attached to the roof substrate with an adhesive such as hot asphalt, but some are self-adhering with the bottom of the sheet and heated with a blow torch.
3- Built up Roofs (BUR's)
Of all flat-roof systems, BUR's are typically the most common. This is partly because of their lower cost and durability. They are considered multiple layer systems because they consist of alternating layers of felt and asphalt. Like the Roll Asphalt roof, they are covered with gravel-like ballast.
4- Modified Bitumen
This is a system made up of modified bituminous membranes, a blend of asphalt and polymers that appear from surface characteristics to be single-ply, except that this membrane is covered with ballast.
5- Flat-Seamed Metal
Although these systems are more commonly used on pitched roofs, they are at times used as flat-roof systems. Composed of thin sheets of metal such as copper, stainless steel, or lead-coated copper, these sheets are soldered together at their edges to prevent moisture penetration of water. Their systems are manufactured to take on the appearance of tile or shingles and are typically preferred because of their aesthetic qualities.